Frequency drift as a result of thermal activity in a cell phone essentially affects all Global Positioning System (GPS) enabled cellular phones and their ability to quickly provide a location fix. Accurately predicting frequency drift rate in parts per million per second (ppm/sec) or parts per billion per second (ppb/sec) is difficult in a typical application due to dynamic signal conditions in a real life environment and the different rates of change of temperature experienced by the phone. The thermal factors typically experienced by a phone can be heavily dependent on ambient temperature, phone temperature, phone transmitter power, relative placement of the crystal (XTAL) or temperature corrected crystal oscillator (TCXO) to the heat generating components in the layout, charger activity, phone mode of operation (emergency call, idle, packet data, etc). Typically, an assumption is made by the software in the phone that accounts for the worst possible thermal drift rate. This assumption leads to longer GPS time to first fix (TTFF) times as the frequency search algorithms must be wide enough to account for these worst case conditions.
Several companies discuss the use of Automatic Frequency Control (AFC) from the cell phone system to provide either a one time assist to the GPS engine or a continuous correction. No existing phone tries to predict or estimate frequency drift rate and drift rate uncertainty of the reference oscillator in the phone. As mentioned above, this drift rate will be different depending on the environmental and phone state.